The issue of changing rates and patterns of salinity in Florida Bay is central to the question of restoration of the South Florida Ecosystem. It is critical to establish what pre-development rates and patterns of change in salinity were, and to decouple natural components of change from human-induced changes during this century.

The US Geological Survey Ecosystem History Projects, using multi-disciplinary and multi-proxy methods, have identified the long-term patterns of salinity change in seven cores taken at four sites in the central and eastern portions of Florida Bay (Bob Allen mudbank, Russell Bank, Pass Key, and the mouth of Taylor Creek). All cores were dated using ²¹⁰Pb geochronology. Our initial results were based on percent abundance of benthic foraminifers, ostracodes, and molluscs in down-core assemblages that were categorized based on the known preferences of a species for a specific salinity range. Evidence from the seven cores indicated that fluctuations in salinity are part of the natural cycle; however, there is a distinct difference in the patterns of fluctuation seen in the benthic salinity indicators pre-1900, compared to post-1940. Subtle shifts in the faunal distributions occur around 1910, but after 1940 the pattern of salinity fluctuation departs substantially from the pre-1900 pattern. The amplitude of shifts in salinity indicators increased from 15-20 % about the mean pre-1900 to 40-60 % post-1940. Analyses of replicate cores from three sites (Bob Allen, Russell, and Pass) were consistent with the initial data.

In conjunction with faunal analyses, stable isotopic analyses were conducted on select mollusc species (Transennella and Brachiodontes) from the same cores. These analyses indicate that between 1900 and 1940, ¹⁸O increased slightly, reflecting increased salinity and/or evaporation. These findings are in agreement with the faunal analyses. The ¹³C increased significantly between 1900 and 1940, which reflects decreased circulation and/or longer residence time.

To date, diatom analyses have been conducted on two of the cores (Russell and Pass) analyzed for faunal content and stable isotopes. Diatom analyses have indicated that significant changes in salinity occurred over the last century, and that periods of high salinity did exist prior to significant human-induced changes. Diatom indicators show the most significant changes in salinity after approximately 1972.

Metal/Ca ratios of ostracodes from three cores (Bob Allen, Russell, and Taylor) have indicated decadal-scale oscillations in salinity, which correlate with averaged decadal-scale fluctuations in annual and seasonal rainfall over southern Florida. These data suggest that regional climate and precipitation are important factors in controlling salinity variations in Florida Bay. Winter rainfall in southern Florida is significantly controlled by the SOI (Southern Oscillation Index - a useful measure of ENSO, El Niño Southern Oscillation strength). Years with strong negative SOI values correspond to anomalously high winter rainfall in southern Florida, and the reverse is true for strongly positive SOI values. Periods of high rainfall correspond to salinity minima and low rainfall to salinity maxima as estimated from the ostracode shell chemistry. It is significant, however, that the highest salinity shifts recorded in metal/Ca ratios occurred in the last 50 years, a period when there is no obvious concomitant change in regional rainfall. Thus, the natural cycle of salinity fluctuations and the correspondence to ENSO and rainfall appears to be decoupled in the second half of this century, implying that other forces are influencing salinity patterns in Florida Bay. This is consistent with the hypothesis that human-induced alteration of the Everglades and the resulting altered delivery of freshwater to the Bay have changed the timing and fluctuation of salinity patterns within the Bay.

The debate over natural versus human-induced changes in the South Florida Ecosystem continues, even as restoration begins. It is critical that these questions be answered, because it is neither economically feasible nor sustainable to attempt to "fix" changes due to natural causes. However, it is feasible and sustainable to restore the components of change due to human-induced alteration of the environment. The data presented here indicate that beginning somewhere around 1935 to 1940, unprecedented changes occurred in the South Florida Ecosystem that affected the natural patterns of salinity fluctuation within Florida Bay. These data agree with results from Smith et al. (1989, Bulletin of Marine Science, v. 44, p. 274-282). Their study of fluorescence patterns in a coral from Florida Bay showed a distinct change in the late 1930's and 1940's, which they attributed to a change in water management practices that further reduced and disturbed the natural fresh-water flow into Florida Bay.

A logical next step in addressing the question of causes of salinity change in Florida Bay is to test the conceptual models of predevelopment seasonal salinity and hydrologic flow by using the historical data acquired from the sediment cores. Currently we are developing the technique of using ostracode (see Dwyer abstract, p. 189, this volume) and mollusc shell geochemistry to refine the decadal-scale patterns developed thus far to an annual scale, and to derive data on pre-canal construction seasonal variations in salinity. These data will provide the means to test hydrologic models for predevelopment salinity within Florida Bay, and may be useful in determining whether changes in water-management practices, or climatic factors can explain the unprecedented changes of the second half of this century.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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